The invention relates to a method for producing an electric lamp in accordance with the preamble of patent claim 1.
Laid-open application WO 98/18297 describes a high-pressure discharge lamp for a motor vehicle headlight, whose cylindrical outer bulb is provided with an auxiliary ignition electrode. The auxiliary ignition electrode is applied to the outer bulb as a strip-shaped metallic coating. This coating can also be configured in such a way that it also acts, in addition, as an optical shield or as a shade. This coating is usually applied to the lamp vessel as a PVD coating or by means of an immersion method.
It is the object of the invention to provide a simplified method for applying an optical shield to a cylindrical section of a lamp vessel.
This object is achieved according to the invention by means of the features of patent claim 1. Particularly advantageous designs of the invention are described in the subclaims.
The method according to the invention for producing an electric lamp which has an opaque shield on a cylindrical section of a lamp vessel, the shield being aligned with reference to the luminous means of the lamp, and a transparent window being defined on the cylindrical section of the lamp vessel with a prescribed dimension perpendicular to the cylinder axis, is distinguished in that the shield is produced from at least two prefabricated and preformed opaque films by virtue of the fact that
a first prefabricated and preformed opaque film is fixed in a defined position with reference to the luminous means of the lamp on the cylindrical section of the lamp vessel, and
a second prefabricated and preformed opaque film is aligned with reference to the first film and fixed in such a way on the cylindrical section of the lamp vessel that the transparent window has the prescribed dimension perpendicular to the cylinder axis.
By contrast with the conventional methods, carrying out the method according to the invention requires no vapor-deposition apparatus for applying the optical shield. Instead, the shield is applied to the cylindrical part of the lamp vessel in the form of at least two prefabricated films. The two films are joined such that the desired form of the optical shield is produced on the lamp vessel. Owing to the assembly of the optical shield from a plurality of films, it is possible to compensate for manufacturing tolerances in the diameter of the cylindrical part of the lamp vessel by placing the films in such a way that they more or less overlap one another, depending on how the diameter deviates from a desired value. This ensures that the transparent window defined by the optical shield always has the same width independently of the manufacturing tolerances of the lamp vessel diameter.
The abovementioned films are advantageously designed as wax films or as films consisting of a wax-like material which are provided with a heat resistant colored layer or have added heat resistant colored pigments. The wax films adhere particularly well to the lamp vessel when they are heated to such an extent that the wax softens. They can then be rolled onto the cylindrical part of the lamp vessel relatively easily. In order to fix the optical shield on the lamp vessel, the lamp vessel with the wax films arranged thereon is subjected to heat treatment such that the wax films vaporize. All that remains on the lamp vessel is the heat resistant colored layer.
In accordance with the preferred exemplary embodiment of the invention, the optical shield is of substantially U-shaped design in order to produce the passing beam in a motor vehicle headlight by means of a high pressure discharge lamp. One U-limb of the shield is applied by means of the first film to the cylindrical section of the lamp vessel, and the other U-limb is applied by means of the second film to the cylindrical section of the lamp vessel. In this case, the U-limbs are aligned parallel to the cylinder axis, and the prescribed dimension of the transparent window perpendicular to the cylinder axis is set by the spacing of the U-limbs.